1. Field of the Invention
The present invention relates to navigational directional reference systems, and more specifically to navigational heading reference systems employing interferometric techniques.
2. Description of the Prior Art
Mechanical gryroscopes are commonly used for indicating and controlling the heading of ships and aircraft.
In the wake of advancing technology, various non-mechanical gyroscopes such as optical electromagnetic wave interferometric devices have been proposed.
The devices are based on the pioneering work of G. Sagnac who, inspired by the earlier work of A. A. Michelson, constructed a relatively compact rotatable interferometer with which angular rotation could be measured. Basically, Sagnac provided a closed optical path as depicted in FIG. 1. Light from a source 11 was incident beam into first and second internal beams 15 and 17 respectively. The internal beams were directed onto totally reflecting mirrors 19, 21 and 23 so as to form an optical path in which the internal beam 15 effectively rotates in a clockwise direction and beam 17 effectively rotates in a counter-clockwise direction. The two internal beams are diverted onto an observing and measuring means 25 by the beam-splitting mirror 13.
When such apparatus is rotated in the plane of the paper, the Sagnac interference fringes produced by the interfering beams are shifted. The direction and magnitude of the fringe shift are proportional to the direction and magnitude of the rotation rate. In the original Sagnac experiment the fringe shift was recorded photographically at the observing means 25.
It should be noted that the Sagnac device was responsive not only to rotation about the axis of the optical path, but also responsive to rotation about an axis outside the optical path. In fact, Sagnac's experiments were directed primarily at the measurement of the earth's rotation.
As described in U.S. Pat. No. 3,332,314, A. H. Rosenthal subsequently devised an optical interferometric navigational instrument using Sagnac interferometer techniques, but employing a radiation source such as a laser to provide a practical directional reference instrument.
Typically, Sagnac interferometric navigation instruments such as those envisioned by Rosenthal may employ photoelectric sensors which develop a beat frequency signal when exposed to the interfering output beams from the optical loop. The beat frequency represents the difference in frequency of the two internal signals caused by rotation of the apparatus as may be measured by straightforward techniques such as a frequency meter or pulse counter. The beat frequency thus may be utilized as a direct indication of the total angular velocity of the optical circuit. This total angular velocity, it should be noted, includes a component arising from the rotation of the earth at the latitude where the ship is located as well as a component arising from the ship's heading. In such devices, therefore, the ship'latitude must be determined and taken into account in calculating the desired value of the ship's heading. The interferometric ship's heading reference system of the present invention eliminates this need for latitude correction.
It will be appreciated by those skilled in the art that although the foregoing discussion was directed to a rectangular optical path, other workers in the art have developed Sagnac interferometers with circular or other non-rectangular shapes. Furthermore Sagnac interferometers have been developed using, light conducting materials such as lucite or optical fibers formed into a single circular loop or into a relatively flat coil of several loops in series.